Allow to get the list of Docker network subnets from pillar config

Summary:
Provide a paas_docker.get_subnets command to get a list of subnets,
normally in CIDR notation.

This list can be used for firewall configuration for example. Ref T1463.

Test Plan: New unit tests provided for themodule

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1463

Differential Revision: https://devcentral.nasqueron.org/D1864